Modern switching power supplies are used for powering a wide variety of solid-state equipment, such as computers, televisions, and communication equipments. The ability to perform power switching at high frequencies allows the use of miniaturized inductors, which in turn tends to reduce the weight and volume of the power supply. A limiting factor in the miniaturization of such power supplies is the heat generated during the switching of the power switches of the supply.
When large amounts of power are to be supplied to an equipment, the power drain from the AC power lines or mains can be large. It is very desirable to reduce the load on the AC power lines by minimizing the current required to deliver a given amount of power. It is well known that the current in the AC power lines may not be in-phase with the applied voltage, as a result of reactive load elements. The out-of-phase condition, in turn, results in the flow of currents having a magnitude greater than would be the case for an in-phase condition. These larger currents, in turn, undesirably tend to stress the power transmission system. Thus, it is desirable to correct the power factor of the input power to a power supply.
In some cases, miniaturization of a power supply can be aided by operation of an alternating-voltage to direct-voltage (AC-to-DC) converter from rectified alternating (pulsating direct) voltage directly from the power mains, without significant capacitive filtering. Such operation reduces the required size of a direct voltage storage capacitor which might be associated with the rectifier. In practice, some minimal amount of filtration may be required, without changing the essence of the rectified voltage from pulsating direct voltage to direct voltage. Plot 22 of FIG. 2 illustrates a pulsating direct voltage V_in which exhibits essentially zero-voltage nulls at times indicated as ta, tb, tc, and td. Pulsating direct voltage differs from alternating voltage in that the voltage direction remains the same during all half-cycles of operation, rather than reversing polarity each half-cycle of operation.
When a switching power supply operates from pulsating direct voltage, the input voltage may at times be near zero amplitude, and may at other times, namely during the peak of a voltage half-cycle, have a significant value. It may often occur that the desired direct output voltage, (V_out) 23 of FIG. 2, may be greater than the minimum pulsating direct input voltage, as for example near time ta, between times t2 and t3, between times t4 and t5, and near time td, and at other times, namely in the time intervals t1 to t2, t3 to t4, and t5 to t6, the value of the pulsating direct voltage may exceed the desired output voltage. Consequently, there is a need for a switching power supply capable of operation in a “boost” mode in which the input voltage is “boosted” to the desired direct output voltage level, and in a “buck” mode in which the direct output voltage is less than the value of the pulsating direct voltage.
Improved or alternative switching power supplies are desired.